In refrigeration systems, such as air conditioners and heat pumps, a refrigerant fluid is subjected to at least one basic vapor compression cycle. Briefly described, a basic vapor compression cycle involves a series of manipulations of a refrigerant fluid. First, the fluid is evaporated in an evaporator in a heat exchange relationship with a medium being cooled. Compression of the vaporized fluid occurs in a compressor, and the fluid is condensed in a condenser in heat exchange relation to a cooling medium or medium being heated. Then expansion of the fluid occurs in an expansion valve. Since cooling in this cycle also results in the production of heat, the system can be utilized for cooling or for heating as in a heat pump. Typical refrigeration systems may consist of a simple vapor compression cycle or more than one cycle, such as a compound cycle.
Increase in volumetric capacity, improvement in energy efficiency, and decrease in compression ratio are continuously sought in such refrigeration systems. In large, low pressure systems with high flow rate, and low pressure ratios, centrifugal compressors are preferred (see ISA Monograph Series No. 3, "Centrifugal Compressors", A. Eli Nisenfeld, 1982, page 15). Compared to reciprocating compressors, the centrifugal compressors offer a combination of lower maintenance costs, greater rangeability and less vibration.
In centrifugal chilling systems which usually use conventional refrigerant fluids, the larger the volumetric capacity of a refrigerant employed, the smaller, and generally more economical, the system. Energy efficiency is measured as coefficient of performance (COP), that is, the ratio of cooling provided by the evaporator, divided by the energy input to the compressor of the cycle. Higher energy efficiency is desirable as this parameter compares the required energy input of different systems for a given amount of heating or cooling. A higher energy efficiency means a lower operating cost. Lower compression ratios are also desirable since, generally, the greater this parameter the less efficient is a compressor's operation.
While new designs in hardware continually improve these parameters in specific applications, hardware designers have been limited to a few well characterized and commercially available refrigerant fluids. Such fluids are trichloromonofluoromethane (conventionally called Isotron.RTM. 11, CFC 11 or R11), monochlorodifluoromethane (Isotron 22 or R22) dichlorodifluoromethane (Isotron 12 or R12), 1-chloro-1,1-difluoroethane (Isotron 142b or R142b), and 1,2-dichloro-1,1,2,2-tetrafluoroethane (Isotron 114 or R114).
These refrigerants have proven less than satisfactory primarily because the most popular fluids for use in refrigerant systems, such as automobile air conditioners, are the fully halogenated refrigerants, such as trichloromonofluoromethane (R11) and 1,2-dichloro-1,1,2,2-tetrafluoroethane (R114). These compounds have serious destructive effects on stratospheric ozone which filters carcinogenic ultraviolet radiation. Consequently, the art has focused its attention to producing new refrigerant fluids to minimize such harm, yet achieve the satisfactory results of the fully halogenated fluids. See, e.g., L. J. Wilson, Chemical Week, pp. 15-16 (July 22, 1987).